blank white square appears only when rendering
When part of an image is transparent, a checker board pattern is shown.

With the background set as transparent, you will see this in the render, This can also be visible in the viewport when it is set to render preview.



In evee and cycles, the transparent background setting can be found in the film panel of the render properties.



It is also possible to get the same effect for a single object. For this you need to use a holdout shader for the object, or setup compositing nodes to cut it out.